Positive electrode active material containing lithium composite oxide and covering material, and battery including positive electrode active material

ABSTRACT

A positive electrode active material contains a lithium composite oxide and a covering material. The lithium composite oxide has a crystal structure that belongs to space group Fd-3m. The ration I(111)/I(400) of a first integrated intensity I(111) of a first peak corresponding to a (111) plane to a second integrated intensity I(400) of a second peak corresponding to a (400) plane in an XRD pattern of the lithium composite oxide satisfies 0.05≤I(111)/I(400)≤0.90. The covering material has an electron conductivity of 106 S/m or less.

BACKGROUND 1. Technical Field

The present disclosure relates to a positive electrode active material for batteries and a battery.

2. Description of the Related Art

Japanese Unexamined Patent Application Publication No. 2008-156163 discloses a spinel lithium manganese oxide which has a chemical composition represented by general formula Li_(1+x)M_(y)Mn_(2−x−y)O₄ and in which the maximum particle diameter D₁₀₀ is 15 μm or less, the full width at half maximum of a (400) plane in X-ray diffraction is 0.30 or less, and the ratio I₄₀₀/I₁₁₁ of a peak intensity I₄₀₀ of the (400) plane to a peak intensity I₁₁₁ of a (111) plane is 0.33 or more. Herein, M represents one or more metal elements selected from Al, Co, Ni, Mg, Zr, and Ti, x is in the range of 0≤x≤0.33, and y is in the range of 0≤y≤0.2.

SUMMARY

In one general aspect, the techniques disclosed here feature a positive electrode active material containing a lithium composite oxide and a covering material that covers a surface of the lithium composite oxide. The covering material has an electron conductivity of 10⁶ S/m or less. The lithium composite oxide has a crystal structure that belongs to space group Fd-3m. The ratio I₍₁₁₁₎/I₍₄₀₀₎ of a first integrated intensity (₍₁₁₁₎ of a first peak corresponding to a (111) plane to a second integrated intensity (₍₄₀₀₎ of a second peak corresponding to a (400) plane in an XRD pattern of the lithium composite oxide satisfies 0.05≤I₍₁₁₁₎/I₍₄₀₀₎≤0.90.

General or specific embodiments of the present disclosure may be implemented as a positive electrode active material for batteries, a battery, a method, or any selective combination thereof.

Additional benefits and advantages of the disclosed embodiments will become apparent from the specification and drawings. The benefits and/or advantages may be individually obtained by the various embodiments and features of the specification and drawings, which need not all be provided in order to obtain one or more of such benefits and/or advantages.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional view illustrating a schematic structure of a battery that is an example of a battery according to a second embodiment;

FIG. 2 illustrates an XRD pattern of a positive electrode active material in Example 1

FIG. 3 illustrates the results of an initial charge-discharge test of batteries in Example 1, Example 3, and Comparative Example 1; and

FIG. 4 illustrates the results of a charge-discharge cycle test of batteries in Example 1, Example 3, and Comparative Example 1.

DETAILED DESCRIPTION

Hereafter, embodiments of the present disclosure will be described.

First Embodiment

A positive electrode active material according to a first embodiment contains a lithium composite oxide and a covering material that covers a surface of the lithium composite oxide and has an electron conductivity of 10⁶ S/m or less. The lithium composite oxide has a crystal structure that belongs to space group Fd-3m, and an integrated intensity ratio I₍₁₁₁₎/I₍₄₀₀₎ of a peak of a (111) plane to a peak of a (400) plane in an XRD pattern of the lithium composite oxide satisfies 0.05≤I₍₁₁₁₎/I₍₄₀₀₎≤0.90.

This configuration can provide batteries having good cycle characteristics.

The term “batteries having good cycle characteristics” refers to batteries having a high capacity retention even after a charge-discharge cycle is repeatedly performed a plurality of times. In other words, the batteries having good cycle characteristics are batteries whose capacity does not considerably decrease even after a charge-discharge cycle is repeatedly performed a plurality of times.

In the case where, for example, a lithium ion battery is produced using the positive electrode active material according to the first embodiment, the lithium ion battery has an oxidation-reduction potential (based on Li/Li⁺) of about 3.4 V. In the lithium ion battery, the capacity retention after 20 cycles of a charge-discharge test is about 80% or more.

The lithium composite oxide according to the first embodiment has a crystal structure that belongs to space group Fd-3m, and the integrated intensity ratio I₍₁₁₁₎/I₍₄₀₀₎ of a peak of a (111) plane to a peak of a (400) plane in an X-ray diffraction (XRD) pattern of the lithium composite oxide satisfies 0.05≤I₍₁₁₁₎/I₍₄₀₀₎≤0.90.

The ratio I₍₁₁₁₎/I₍₄₀₀₎ is a parameter that is an index of cation mixing in a lithium composite oxide having a crystal structure that belongs to space group Fd-3m. The term “cation mixing” in the present disclosure refers to a state in which a lithium atom and a cation atom such as a transition metal are replaced with each other in a crystal structure of the lithium composite oxide. As the degree of cation mixing decreases, the ratio I₍₁₁₁₎/I₍₄₀₀₎ increases. As the degree of cation mixing increases, the ratio I₍₁₁₁₎/I₍₄₀₀₎ decreases.

Since the lithium composite oxide according to the first embodiment has a crystal structure that belongs to space group Fd-3m and 0.05≤I₍₁₁₁₎/I₍₄₀₀₎≤0.90 is satisfied, the degree of cation mixing is believed to be relatively high. Therefore, the lithium atom and the cation atom such as a transition metal are believed to be subjected to cation mixing at all of “8a sites, 16d sites, and 16c sites” corresponding to “cation sites” (i.e., “Li layer” and “transition metal layer”). As long as 0.05≤I₍₁₁₁₎/I₍₄₀₀₎≤0.90 is satisfied, the Li occupancy at each of the sites is not limited.

In addition to high diffusibility of Li in the “Li layer”, the above-described cation mixing improves the diffusibility of Li in the “transition metal layer”. Furthermore, the diffusibility of Li between the “Li layer” and the “transition metal layer” is improved. That is, Li can efficiently diffuse throughout the “cation sites”. Therefore, the lithium composite oxide according to the first embodiment is more suitable for producing high-capacity batteries than known ordered lithium composite oxides (i.e., the degree of cation mixing is low).

If the ratio I₍₁₁₁₎/I₍₄₀₀₎ of the lithium composite oxide according to the first embodiment is less than 0.05, the Li occupancy excessively increases in the “transition metal layer” and thus the crystal structure becomes thermodynamically unstable. Therefore, the crystal structure is broken when the deintercalation of Li during charge proceeds, which causes an insufficient capacity.

If the ratio I₍₁₁₁₎/I₍₄₀₀₎ of the lithium composite oxide according to the first embodiment is more than 0.90, the cation mixing is suppressed and thus the Li occupancy in the “transition metal layer” decreases, which reduces the number of three-dimensional diffusion paths of Li. Consequently, the diffusibility of Li degrades, which causes an insufficient capacity.

In the lithium composite oxide according to the first embodiment, 0.05≤I₍₁₁₁₎/I₍₄₀₀₎≤0.90 is satisfied and thus the lithium atom and the cation atom such as a transition metal are believed to be sufficiently subjected to cation mixing. Therefore, it is believed that the number of three-dimensional diffusion paths of lithium increases in the lithium composite oxide according to the first embodiment and thus a larger amount of Li can be intercalated and deintercalated.

Since the lithium composite oxide according to the first embodiment has a crystal structure that belongs to space group Fd-3m and 0.05≤I₍₁₁₁₎/I₍₄₀₀₎≤0.90 is satisfied, a transition metal-anion octahedron serving as a pillar three-dimensionally forms a network even when a large amount of Li is extracted, and thus the crystal structure can be stably maintained. Therefore, it is believed that the positive electrode active material according to the first embodiment can intercalate and deintercalate a larger amount of Li. That is, the positive electrode active material according to the first embodiment is suitable for providing high-capacity batteries. For the same reason, the positive electrode active material according to the first embodiment is also suitable for providing batteries having good cycle characteristics.

It is believed that, for example, the layered crystal structure that belongs to space group Fd-3m is easily maintained and is not easily broken even when a large amount of Li is extracted compared with layered structures that belong to space group R-3m.

Herein, for example, Japanese Unexamined Patent Application Publication No. 2008-156163 is taken as a comparative example. Japanese Unexamined Patent Application Publication No. 2008-156163 discloses a positive electrode material containing a lithium composite oxide which has a crystal structure that belongs to space group Fd-3m and in which a lithium atom and a cation atom such as a transition metal are not sufficiently subjected to cation mixing. This lithium composite oxide approximately satisfies 2≤I₍₁₁₁₎/I₍₄₀₀₎≤3. It is also described that the disorder of the crystal structure is extremely reduced and such a positive electrode material has good battery characteristics.

That is, in the related art, a lithium composite oxide that satisfies 0.05≤I₍₁₁₁₎/I₍₄₀₀₎≤0.90 has not been investigated or suggested.

In contrast, the lithium composite oxide according to the first embodiment has a crystal structure that belongs to space group Fd-3m and satisfies 0.05≤I₍₁₁₁₎/I₍₄₀₀₎≤0.90. Thus, the present inventors have realized a high-capacity battery that is beyond expectation in the related art.

When a typical positive electrode active material is used at a high potential, the decomposition (e.g., side reaction) of an electrolyte is facilitated, which generates a resistance layer. Furthermore, when a typical positive electrode active material is used at a high potential, anions contained in the positive electrode active material separate in the form of gas. This may degrade the cycle characteristics.

The positive electrode active material according to the first embodiment further contains a covering material in addition to the above-described lithium composite oxide. The covering material covers a surface of the lithium composite oxide. The covering material has an electron conductivity of 10⁶ S/m or less and is a material different from the lithium composite oxide. When the covering material covers a surface of the lithium composite oxide, the contact between the lithium composite oxide and the electrolyte is suppressed. The low electron conductivity of the covering material suppresses the transfer of electrons between the lithium composite oxide and the electrolyte, and the side reactions caused by the transfer of electrons can be suppressed. Thus, the generation of a resistance layer or the separation in the form of gas is suppressed. Therefore, batteries having good cycle characteristics can be provided.

That is, even when the positive electrode active material according to the first embodiment is used at a high potential, batteries having good cycle characteristics can be provided.

In the present disclosure, the phrase “cover a surface” includes a state in which a surface is completely covered and a state in which a surface is partly covered.

In the first embodiment, the mass ratio of the covering material to the above-described lithium composite oxide may be 0.2 or less.

This configuration can suppress the electrochemical inactivation of the surface of the positive electrode active material according to the first embodiment. This can suppress an increase in the resistance. Thus, batteries having higher capacity and better cycle characteristics can be provided.

In the first embodiment, the mass ratio of the covering material to the above-described lithium composite oxide may be 0.01 or more and 0.1 or less.

This configuration can provide batteries having higher capacity and better cycle characteristics.

The covering material according to the first embodiment may cover a surface of the above-described lithium composite oxide.

This configuration can provide batteries having better cycle characteristics.

The phrase “cover a surface” includes a state in which a surface is completely covered and a state in which a surface is partly covered.

The covering material according to the first embodiment may cover a surface of the above-described lithium composite oxide with a thickness of 0.1 nm or more and 2.0 nm or less.

This configuration can provide batteries having higher capacity and better cycle characteristics.

When the covering material has a thickness of 0.1 nm or more, the uniformity of the thickness can be improved. Furthermore, when the covering material has a thickness of 0.1 nm or more, the dielectric breakdown can be suppressed.

When the covering material has a thickness of 2.0 nm or less, the inhibition of Li conduction due to the covering material can be suppressed.

The covering material according to the first embodiment may chemically modify a surface of the above-described lithium composite oxide.

The covering material according to the first embodiment may form a solid solution with at least a part of the surface of the above-described lithium composite oxide.

This configuration can provide batteries having better cycle characteristics because elution (e.g., elimination) of metal elements can be further suppressed.

The covering material according to the first embodiment has an electron conductivity of 10⁶ S/m or less.

This configuration can provide batteries having better cycle characteristics.

The covering material according to the first embodiment is, for example, an inorganic material.

The covering material according to the first embodiment may be one or more materials selected from the group consisting of graphite, carbon black, and graphite fluoride. Examples of the carbon black include acetylene black, Ketjenblack (registered trademark), channel black, furnace black, lamp black, and thermal black.

The covering material may be an insulating compound. The covering material does not necessarily contribute to an electrochemical reaction.

The covering material according to the first embodiment may be at least one material selected from the group consisting of oxides, halogen compounds, sulfides, and phosphorus compounds.

For example, the halogen compound may be AlF₃.

The covering material according to the first embodiment may be a lithium ion conductor.

For example, the lithium ion conductor may be one or more materials selected from the group consisting of LiBO₂, Li₃PO₄, LiNbO₃, LiNbO₂, LiAlO₂, Li₂SO₄, Li₂MoO₄, Li₄SiO₄, Li₄FeO₄, Li₄ZrO₄, Li₂CO₃, LiW₂O₇, Li₃VO₄, LiCl, LiBr, LiI, Li₂Se, and various lithium ion conductive glasses such as Li₂O—B₂O₃, Li₂O—Al₂O₃, Li₂O—SiO₄, Li₂O—P₂O₅, LiF—BF₃, LiF—AlF₃, and LiF-VF₃.

This configuration can provide batteries having better cycle characteristics.

The covering material according to the first embodiment may be a Li-containing oxide having high Li conductivity.

For example, the Li-containing oxide may be one or more oxides selected from the group consisting of lithium borate, lithium niobate, lithium cobaltate, lithium titanate, and lithium aluminate.

In this configuration, the diffusibility of Li ions is improved and thus batteries having higher capacity can be provided.

When the covering material according to the first embodiment is capable of occluding Li during charge-discharge reaction, batteries having higher capacity can be provided. In this case, the covering material has a charge-discharge capacity. Therefore, even if the mass ratio of the lithium composite oxide to the entire positive electrode active material is decreased by adding the covering material, high capacity of the battery is maintained.

The covering material according to the first embodiment may be an oxide.

This configuration can provide batteries having better cycle characteristics. Oxides are chemically stable and have low reactivity with an electrolyte. Therefore, it is believed that by using oxides, the crystal structure is easily maintained in an electrochemical reaction.

The covering material according to the first embodiment may be an oxide represented by composition formula (1) below.

Li_(a)A_(b)O_(c)   formula (1)

Herein, A may represent one or more elements selected from the group consisting of Mn, Co, Ni, Fe, Cu, Nb, Mo, Ti, Al, Cr, Zr, Zn, Na, K, Ca, Mg, Pt, Au, B, P, Eu, Sm, Ce, and H.

Furthermore, the following conditions may be satisfied:

0≤a≤3,

0.5≤b≤4, and

1≤c≤4.

This configuration can provide batteries having better cycle characteristics.

The covering material according to the first embodiment may be, for example, one or more oxides selected from the group consisting of MnO₂, Al₂O₃, MgO, ZrO₂, ZnO, TiO₂, H₃BO₃, Mn₂O₃, Fe₂O₃, CuO, NiO, Co₃O₄, Eu₂O₃, Sm₂O₃, and CeO₂.

This configuration can provide batteries having better cycle characteristics.

The covering material according to the first embodiment may be one or more oxides selected from the group consisting of Al₂O₃, ZrO₂, ZnO, and TiO₂.

This configuration can provide batteries having better cycle characteristics.

The covering material according to the first embodiment may be a transition metal oxide.

This configuration can provide batteries having better cycle characteristics.

The covering material according to the first embodiment may contain the same metal element as that contained in the above-described lithium composite oxide.

In this configuration, the metal elements firmly bond to each other through, for example, formation of a solid solution at an interface between the lithium composite oxide and the covering material. Therefore, the elution (e.g., elimination) of the metal element is suppressed. This can provide batteries having better cycle characteristics.

The lithium composite oxide according to the first embodiment may satisfy 0.05≤I₍₁₁₁₎/I₍₄₀₀₎≤0.70.

This configuration can provide batteries having higher capacity.

The lithium composite oxide according to the first embodiment may satisfy 0.05≤I₍₁₁₁₎/I₍₄₀₀₎≤0.30.

This configuration can provide batteries having higher capacity.

Typically, peaks of a (111) plane and a (400) plane in a CuKα XRD pattern are present at diffraction angles 20 of 18° to 20° and 44° to 46°, respectively.

The integrated intensity of each diffraction peak can be calculated using, for example, software included with an XRD instrument (e.g., PDXL included with a powder X-ray diffraction instrument manufactured by Rigaku Corporation). In this case, the integrated intensity of each diffraction peak can be determined by, for example, calculating an area within ±3° of an angle of each diffraction peak.

In the lithium composite oxide according to the first embodiment, the element constituting the above-described “transition metal layer” may be a cation atom such as a transition metal. The cation atom such as a transition metal may be, for example, one or more elements selected from the group consisting of Mn, Co, Ni, Fe, Cu, V, Nb, Mo, Ti, Cr, Zr, Zn, Na, K, Ca, Mg, Pt, Au, Ag, Ru, W, B, Si, P, and Al.

This configuration can provide batteries having higher capacity.

In the lithium composite oxide according to the first embodiment, the element constituting the above-described “transition metal layer” may be, for example, at least one element selected from the group consisting of Mn, Co, Ni, Fe, Cu, V, Ti, Cr, and Zn, that is, may be at least one 3d transition metal element.

This configuration can provide batteries having higher capacity.

In the lithium composite oxide according to the first embodiment, the element constituting the above-described “transition metal layer” may be, for example, one or more elements selected from the group consisting of Mn, Co, Ni, Fe, Cu, V, Nb, Ti, Cr, Ru, W, B, Si, P, and Al.

This configuration can provide batteries having higher capacity.

The lithium composite oxide according to the first embodiment may contain Mn.

In this configuration, when the lithium composite oxide contains Mn having orbitals easily hybridized with those of oxygen, the elimination of oxygen during charge is suppressed. Furthermore, even when the degree of cation mixing is relatively high (e.g., 0.05≤I₍₁₁₁₎/I₍₄₀₀₎≤0.90 is satisfied), the crystal structure is stabilized. Therefore, it is believed that a larger amount of Li can be intercalated and deintercalated. This can provide batteries having higher capacity.

The lithium composite oxide according to the first embodiment may contain one or more elements selected from the group consisting of F, Cl, N, and S.

In this configuration, the crystal structure is believed to be stabilized by partly replacing oxygen with an electrochemically inactive anion. It is also believed that crystal lattice expansion is caused by partly replacing oxygen with an anion having a large ionic radius, which improves the diffusibility of Li. Furthermore, even when the degree of cation mixing is relatively high (e.g., 0.05≤I₍₁₁₁₎/I₍₄₀₀₎≤0.90 is satisfied), the crystal structure is stabilized. Therefore, it is believed that a larger amount of Li can be intercalated and deintercalated. This can provide batteries having higher capacity.

The lithium composite oxide according to the first embodiment may contain F.

In this configuration, the cation-anion interaction is increased by partly replacing oxygen with F, which has a high electronegativity, thereby improving the discharge capacity or operating voltage of the battery. Furthermore, by dissolving F, which has a high electronegativity, electrons are localized compared with in the case where F is not contained. This can suppress the elimination of oxygen during charge and thus the crystal structure is stabilized. Furthermore, even when the degree of cation mixing is relatively high (e.g., 0.05≤I₍₁₁₁₎/I₍₄₀₀₎≤0.90 is satisfied), the crystal structure is stabilized. Therefore, it is believed that a larger amount of Li can be intercalated and deintercalated. By combining these effects, batteries having higher capacity can be provided.

Next, an example of the chemical composition of the lithium composite oxide according to the first embodiment will be described.

The lithium composite oxide according to the first embodiment may be a compound represented by composition formula (2) below.

Li_(x)Me_(y)O_(α)Q_(β)  formula (2)

Herein, Me may represent one or more elements selected from the group consisting of Mn, Co, Ni, Fe, Cu, V, Nb, Mo, Ti, Cr, Zr, Zn, Na, K, Ca, Mg, Pt, Au, Ag, Ru, W, B, Si, P, and Al.

Me may include at least one element selected from the group consisting of Mn, Co, Ni, Fe, Cu, V, Ti, Cr, and Zn, that is, may include at least one 3d transition metal element.

Q may represent one or more elements selected from the group consisting of F, Cl, N, and S.

Furthermore, the following conditions may be satisfied in the composition formula (2):

1.05≤x≤1.4,

0.6≤y≤0.95,

1.33≤α≤2, and

0≤β≤0.67.

This configuration can provide batteries having higher capacity.

In the first embodiment, when Me is constituted by two or more elements (e.g., Me′ and Me″) and the composition ratio is “Me′_(y1)Me″_(y2)”, “y=y1+y2” is given. For example, when Me is constituted by two elements (Mn and Co) and the composition ratio is “Mn_(0.6)Co_(0.2)”, “y=0.6+0.2=0.8” is given. In the case where Q is constituted by two or more elements, the calculation can be conducted in the same manner as in the case of Me.

When x is 1.05 or more in the compound represented by the composition formula (2), the amount of Li that can be used is increased. This increases the capacity.

When x is 1.4 or less in the compound represented by the composition formula (2), the oxidation-reduction reaction of Me can be more utilized. As a result, the oxidation-reduction reaction of oxygen is less utilized. Thus, the crystal structure is stabilized. This increases the capacity.

When y is 0.6 or more in the compound represented by the composition formula (2), the oxidation-reduction reaction of Me can be more utilized. As a result, the oxidation-reduction reaction of oxygen is less utilized. Thus, the crystal structure is stabilized. This increases the capacity.

When y is 0.95 or less in the compound represented by the composition formula (2), the amount of Li that can be used is increased. This increases the capacity.

When α is 1.33 or more in the compound represented by the composition formula (2), the amount of charge compensation due to the oxidation-reduction reaction of oxygen is increased. This increases the capacity.

When α is 2 or less in the compound represented by the composition formula (2), an excess increase in the capacity due to the oxidation-reduction reaction of oxygen can be prevented, which stabilizes a structure when Li is deintercalated. This increases the capacity.

When β is 0.67 or less in the compound represented by the composition formula (2), the influence of Q that is electrochemically inactive is reduced, which improves the electron conductivity. This increases the capacity.

In the compound represented by the composition formula (2), Me may include one or more elements selected from the group consisting of Mn, Co, Ni, Fe, Cu, V, Nb, Ti, Cr, Ru, W, B, Si, P, and Al.

This configuration can provide batteries having higher capacity.

In the compound represented by the composition formula (2), Me may include Mn.

That is, Me may represent Mn.

Alternatively, Me may include Mn and one or more elements selected from the group consisting of Co, Ni, Fe, Cu, V, Nb, Mo, Ti, Cr, Zr, Zn, Na, K, Ca, Mg, Pt, Au, Ag, Ru, W, B, Si, P, and Al.

In this configuration, when Me includes Mn having orbitals easily hybridized with those of oxygen, the elimination of oxygen during charge is suppressed. Furthermore, even when the degree of cation mixing is relatively high (e.g., 0.05≤I₍₁₁₁₎/I₍₄₀₀₎≤0.90 is satisfied), the crystal structure is stabilized. This can provide batteries having higher capacity.

In the compound represented by the composition formula (2), the Mn content in Me may be 50 mol % or more. That is, the molar ratio (Mn/Me ratio) of Mn to Me including Mn may be 0.5 to 1.0.

In this configuration, when Me sufficiently includes Mn having orbitals easily hybridized with oxygen, the elimination of oxygen during charge is further suppressed. Furthermore, even when the degree of cation mixing is relatively high (e.g., 0.05≤I₍₁₁₁₎/I₍₄₀₀₎≤0.90 is satisfied), the crystal structure is further stabilized. This can provide batteries having higher capacity.

In the compound represented by the composition formula (2), the Mn content in Me may be 75 mol % or more. That is, the molar ratio (Mn/Me ratio) of Mn to Me including Mn may be 0.75 to 1.0.

In this configuration, when Me includes Mn having orbitals easily hybridized with those of oxygen in a larger amount, the elimination of oxygen during charge is further suppressed. Furthermore, even when the degree of cation mixing is relatively high (e.g., 0.05≤I₍₁₁₁₎/I₍₄₀₀₎≤0.90 is satisfied), the crystal structure is further stabilized. This can provide batteries having higher capacity.

In the compound represented by the composition formula (2), Me may include 20 mol % or less of one or more elements selected from the group consisting of B, Si, P, and Al relative to Me.

In this configuration, the structure is stabilized by adding an element that readily forms covalent bonds, which improves the cycle characteristics. This can provide batteries having a longer life.

The compound represented by the composition formula (2) may satisfy the following conditions:

1.1≤x≤1.2, and

y=0.8.

This configuration can provide batteries having higher capacity.

The compound represented by the composition formula (2) may satisfy the following conditions:

1.67≤α≤2, and

0≤β≤0.33.

This configuration can provide batteries having higher capacity.

The compound represented by the composition formula (2) may satisfy the following conditions:

1.67≤α<2, and

0<β≤0.33.

This configuration can provide batteries having higher capacity.

That is, the compound represented by the composition formula (2) may contain Q (i.e., one or more elements selected from the group consisting of F, Cl, N, and S).

In this configuration, the crystal structure is believed to be stabilized by partly replacing oxygen with an electrochemically inactive anion. It is also believed that crystal lattice expansion is caused by partly replacing oxygen with an anion having a large ionic radius, which improves the diffusibility of Li. Furthermore, even when the degree of cation mixing is relatively high (e.g., 0.05≤I₍₁₁₁₎/I₍₄₀₀₎≤0.90 is satisfied), the crystal structure is stabilized. This can provide batteries having higher capacity.

In the compound represented by the composition formula (2), Q may include F.

That is, Q may represent F.

Alternatively, Q may include F and one or more elements selected from the group consisting of Cl, N, and S.

In this configuration, the cation-anion interaction is increased by partly replacing oxygen with F, which has a high electronegativity, thereby improving the discharge capacity or operating voltage of the battery. Furthermore, by dissolving F, which has a high electronegativity, electrons are localized compared with in the case where F is not contained. This can suppress the elimination of oxygen during charge and thus the crystal structure is stabilized. Furthermore, even when the degree of cation mixing is relatively high (e.g., 0.05≤I₍₁₁₁₎/I₍₄₀₀₎≤0.90 is satisfied), the crystal structure is stabilized. By combining these effects, batteries having higher capacity can be provided.

The compound represented by the composition formula (2) may satisfy the following conditions:

1.67≤α≤1.9, and

0.1≤β≤0.33.

This configuration can provide batteries having higher capacity.

In the compound represented by the composition formula (2), the ratio of “Li” and “Me” is represented by x/y.

The compound represented by the composition formula (2) may satisfy 1.3≤x/y≤1.9.

This configuration can provide batteries having higher capacity.

When x/y is more than 1, the ratio of the number of Li atoms at sites at which Li atoms are located is larger than that of a known positive electrode active material represented by, for example, composition formula LiMnO₂. This allows a larger amount of Li to be intercalated and deintercalated.

When x/y is 1.3 or more, a large amount of Li can be used and thus the diffusion paths of Li can be appropriately formed. This can provide batteries having higher capacity.

When x/y is 1.9 or less, less utilization of the oxidation-reduction reaction of Me can be prevented. As a result, the oxidation-reduction reaction of oxygen is less utilized. Furthermore, the crystal structure can be prevented from becoming unstable when Li is deintercalated during charge, and the efficiency of intercalating Li during discharge can be prevented from decreasing. This can provide batteries having higher capacity.

The compound represented by the composition formula (2) may satisfy 1.38≤x/y≤1.5.

This configuration can provide batteries having higher capacity.

In the compound represented by the composition formula (2), the ratio of “O” and “Q” is represented by α/β.

The compound represented by the composition formula (2) may satisfy 5≤α/β≤19.

This configuration can provide batteries having higher capacity.

When α/β is 5 or more, the amount of charge compensation due to the oxidation-reduction reaction of oxygen can be prevented from decreasing. Furthermore, the influence of Q that is electrochemically inactive can be reduced and thus the electron conductivity is improved. This can provide batteries having higher capacity.

When α/β is 19 or less, an excess increase in the capacity due to the oxidation-reduction reaction of oxygen can be prevented, which stabilizes a structure when Li is deintercalated. Furthermore, the influence of Q that is electrochemically inactive stabilizes a structure when Li is deintercalated. This can provide batteries having higher capacity.

In the compound represented by the composition formula (2), the ratio of “Li+Me” and “O+Q” (i.e., the ratio of “cation” and “anion”) is represented by (x+y)/(α+β).

The compound represented by the composition formula (2) may satisfy 0.75≤(x+y)/(α+β)≤1.2.

This configuration can provide batteries having higher capacity.

When (x+y)/(α+β) is 0.75 or more, the generation of a large amount of impurities due to phase separation during synthesis can be prevented. This can provide batteries having higher capacity.

When (x+y)/(α+β) is 1.2 or less, a structure with a small amount of anion deficiency is formed, and thus the crystal structure is stabilized when Li is deintercalated during charge. This can provide batteries having higher capacity.

The compound represented by the composition formula (2) may satisfy 0.95≤(x+y)/(α+β)≤1.0.

This configuration can provide batteries having higher capacity and better cycle characteristics.

When (x+y)/(α+β) is 1.0 or less, a cation-deficient structure is formed, and thus a larger number of Li diffusion paths are formed. This can provide batteries having higher capacity. Furthermore, cation deficits are randomly arranged in an initial state, and thus the structure is not destabilized when Li is deintercalated. This can provide batteries having good cycle characteristics and a long life.

In the lithium composite oxide according to the first embodiment, Li may be partly replaced with an alkali metal such as Na or K.

The positive electrode active material according to the first embodiment may contain the above-described lithium composite oxide as a main component (i.e., 50% or more relative to the entire positive electrode active material on a mass basis (50 mass % or more)).

This configuration can provide batteries having higher capacity.

The positive electrode active material according to the first embodiment may contain 70% or more (70 mass % or more) of the lithium composite oxide relative to the entire positive electrode active material on a mass basis.

This configuration can provide batteries having higher capacity.

The positive electrode active material according to the first embodiment may contain 90% or more (90 mass % or more) of the lithium composite oxide relative to the entire positive electrode active material on a mass basis.

This configuration can provide batteries having higher capacity.

The positive electrode active material according to the first embodiment may further contain unavoidable impurities in addition to the above-described lithium composite oxide and covering material.

The positive electrode active material according to the first embodiment may further contain, in addition to the above-described lithium composite oxide, at least one selected from the group consisting of a starting material used when the positive electrode active material is synthesized, a by-product, and a decomposition product.

The positive electrode active material according to the first embodiment may contain only the above-described lithium composite oxide and covering material except for, for example, unavoidable impurities.

This configuration can provide batteries having higher capacity and better cycle characteristics.

Method for Producing Compound

Hereafter, an example of a method for producing a lithium composite oxide contained in the positive electrode active material according to the first embodiment will be described.

The lithium composite oxide according to the first embodiment can be produced by, for example, the following method.

A raw material containing Li, a raw material containing Me, and a raw material containing Q are provided.

Examples of the raw material containing Li include oxides such as Li₂O and Li₂O₂; salts such as LiF, Li₂CO₃, and LiOH; and lithium composite oxides such as LiMeO₂ and LiMe₂O₄.

Examples of the raw material containing Me include oxides such as Me₂O₃, salts such as MeCo₃ and MeNO₃, hydroxides such as Me(OH)₂ and MeOOH, and lithium composite oxides such as LiMeO₂ and LiMe₂O₄.

For example, when Me represents Mn, examples of the raw material containing Mn include manganese oxides such as MnO₂ and Mn₂O₃, salts such as MnCo₃ and MnNO₃, hydroxides such as Mn(OH)₂ and MnOOH, and lithium composite oxides such as LiMnO₂ and LiMn₂O₄.

Examples of the raw material containing Q include lithium halides, transition metal halides, transition metal sulfides, and transition metal nitrides.

For example, when Q represents F, examples of the raw material containing F include LiF and transition metal fluorides.

These raw materials are weighed so as to have, for example, the molar ratio in the composition formula (2).

Thus, “x, y, α, and β” in the composition formula (2) can be changed within the range in the composition formula (2).

The weighed raw materials are mixed with each other by, for example, a dry process or a wet process and mechanochemically reacted for 10 hours or more to obtain a compound (precursor). For example, a mixer such as a planetary ball mill can be used.

Then, the obtained compound is heat-treated. This provides a structure in which atoms are partly ordered and thus the lithium composite oxide according to the first embodiment is obtained.

The heat treatment conditions are appropriately set so that the lithium composite oxide according to the first embodiment is obtained. Although the optimum heat treatment conditions vary depending on other production conditions and the target composition, the present inventors have found that the degree of cation mixing tends to decrease as the heat treatment temperature increases or as the heat treatment time increases. Therefore, manufacturers can determine the heat treatment conditions on the basis of this tendency. The heat treatment temperature may be selected from, for example, the range of 200° C. to 600° C. and the heat treatment time may be selected from, for example, the range of 5 minutes to 20 hours. The atmosphere during the heat treatment may be an air atmosphere, an oxide atmosphere, or an inert atmosphere such as a nitrogen or argon atmosphere.

By controlling the raw materials used, the mixing conditions of the raw material mixture, and the heat treatment conditions as described above, the lithium composite oxide according to the first embodiment can be substantially obtained.

The energy required for mixing the elements can be further decreased by using, for example, a lithium-transition metal composite oxide as a precursor. This provides the lithium composite oxide according to the first embodiment with a higher purity.

The composition of the obtained lithium composite oxide can be determined by, for example, ICP emission spectrometry, an inert gas fusion-infrared absorption method, ion chromatography, or a combination thereof.

The space group of the crystal structure of the obtained lithium composite oxide can be determined by powder X-ray analysis.

As described above, the method for producing a lithium composite oxide according to the first embodiment includes (a) a step of providing raw materials, (b) a step of mechanochemically reacting the raw materials to obtain a precursor of a lithium composite oxide, and (c) a step of heat-treating the precursor to obtain a lithium composite oxide.

The step (a) may include a step of preparing a raw material mixture by mixing the raw materials such that the molar ratio of Li to Me is 1.3 or more and 1.9 or less.

Herein, the step (a) may include a step of producing a lithium compound serving as a raw material by a publicly known method.

The step (a) may include a step of preparing a raw material mixture by mixing the raw materials such that the molar ratio of Li to Me is 1.38 or more and 1.5 or less.

The step (b) may include a step of mechanochemically reacting the raw materials using a ball mill.

As described above, the lithium composite oxide according to the first embodiment can be synthesized by mechanochemically reacting the precursor (e.g., LiF, Li₂O, transition metal oxide, and lithium-transition metal composite oxide) using a planetary ball mill.

When the precursor is reacted by a solid-phase method, a more stable compound having a structure in which Li and Me are ordered is obtained.

That is, for example, when the precursor is reacted by a solid-phase method, a lithium composite oxide that has a crystal structure which belongs to space group Fd-3m and that has a ratio I₍₁₁₁₎/I₍₄₀₀₎ of 0.05 or more and 0.90 or less in an X-ray diffraction pattern is not obtained.

Hereafter, an example of a method for treating a covering material contained in the positive electrode active material according to the first embodiment will be described.

Any treatment method for further adding the covering material according to the first embodiment to the obtained lithium composite oxide may be employed. Examples of the treatment method that may be employed include an atomic layer deposition method, a neutralization reaction, a silane coupling reaction, a sol-gel process, and use of a planetary ball mill.

The covering material according to the first embodiment is, for example, a metal oxide. The metal oxide may be treated by, for example, a neutralization reaction. For example, an acidic salt of metal oxide may be added to an alkaline aqueous solution containing the lithium composite oxide dissolved therein. This causes a neutralization reaction and thus a metal oxide coating film can be formed on the surface of the lithium composite oxide. Examples of the acidic salt of metal oxide include manganese nitrate, magnesium nitrate, aluminum sulfate, calcium nitrate, sodium nitrate, potassium nitrate, and titanium sulfate.

The method for treating a covering material may be, for example, an atomic layer deposition method. Thus, a coating film of the covering material can be formed on the surface of the lithium composite oxide. Examples of the covering material include alumina, titanium oxide, zirconium oxide, zinc oxide, and tantalum oxide.

The presence of the lithium composite oxide and covering material contained in the positive electrode active material according to the first embodiment (e.g., covering of the surface of the lithium composite oxide with the covering material) can be observed by, for example, X-ray photoelectron spectroscopy or use of a scanning electron microscope or a transmission electron microscope.

Second Embodiment

Hereafter, a second embodiment will be described. Note that the same description as in the first embodiment will be appropriately omitted to avoid redundancy.

A battery according to the second embodiment includes a positive electrode containing the positive electrode active material according to the first embodiment, a negative electrode, and an electrolyte.

This configuration can provide batteries having good cycle characteristics.

In the battery according to the second embodiment, the positive electrode may include a positive electrode active material layer. Herein, the positive electrode active material layer may contain the positive electrode active material according to the first embodiment as a main component (i.e., 50% or more relative to the entire positive electrode active material layer on a mass basis (50 mass % or more)).

This configuration can provide batteries having better cycle characteristics.

Alternatively, in the battery according to the second embodiment, the positive electrode active material layer may contain 70% or more (70 mass % or more) of the positive electrode active material according to the first embodiment relative to the entire positive electrode active material layer on a mass basis.

This configuration can provide batteries having better cycle characteristics.

Alternatively, in the battery according to the second embodiment, the positive electrode active material layer may contain 90% or more (90 mass % or more) of the positive electrode active material according to the first embodiment relative to the entire positive electrode active material layer on a mass basis.

This configuration can provide batteries having better cycle characteristics.

The battery according to the second embodiment can be used for, for example, lithium ion secondary batteries, nonaqueous electrolyte secondary batteries, and all-solid-state batteries.

That is, the negative electrode of the battery according to the second embodiment may contain, for example, a negative electrode active material capable of occluding and releasing lithium ions. Alternatively, the negative electrode may contain, for example, a material capable of dissolving and precipitating lithium metal as a negative electrode active material.

In the battery according to the second embodiment, the electrolyte may be, for example, a nonaqueous electrolyte (e.g., nonaqueous electrolytic solution).

In the battery according to the second embodiment, the electrolyte may be, for example, a solid electrolyte.

FIG. 1 is a sectional view illustrating a schematic structure of a battery 10 that is an example of the battery according to the second embodiment.

As illustrated in FIG. 1, the battery 10 includes a positive electrode 21, a negative electrode 22, a separator 14, a case 11, a sealing plate 15, and a gasket 18.

The separator 14 is disposed between the positive electrode 21 and the negative electrode 22.

The positive electrode 21, the negative electrode 22, and the separator 14 are impregnated with, for example, a nonaqueous electrolyte (e.g., nonaqueous electrolytic solution).

The positive electrode 21, the negative electrode 22, and the separator 14 constitute an electrode assembly.

The electrode assembly is accommodated in the case 11.

The case 11 is sealed using the gasket 18 and the sealing plate 15.

The positive electrode 21 includes a positive electrode current collector 12 and a positive electrode active material layer 13 disposed on the positive electrode current collector 12.

The positive electrode current collector 12 is made of, for example, a metal material (e.g., aluminum, stainless steel, or aluminum alloy).

The positive electrode current collector 12 may be omitted, and the case 11 may be used as a positive electrode current collector.

The positive electrode active material layer 13 contains the positive electrode active material according to the first embodiment.

The positive electrode active material layer 13 may optionally contain, for example, additives (e.g., a conductive agent, an ion conductive auxiliary agent, and a binding agent).

The negative electrode 22 includes a negative electrode current collector 16 and a negative electrode active material layer 17 disposed on the negative electrode current collector 16.

The negative electrode current collector 16 is made of, for example, a metal material (e.g., aluminum, stainless steel, or aluminum alloy).

The negative electrode current collector 16 may be omitted, and the sealing plate 15 may be used as a negative electrode current collector.

The negative electrode active material layer 17 contains a negative electrode active material.

The negative electrode active material layer 17 may optionally contain, for example, additives (a conductive agent, an ion conductive auxiliary agent, and a binding agent).

Examples of the negative electrode active material include metal materials, carbon materials, oxides, nitrides, tin compounds, and silicon compounds.

The metal material may be a single metal or an alloy. Examples of the metal material include lithium metal and lithium alloys.

Examples of the carbon material include natural graphite, coke, partially graphitized carbon, carbon fiber, spherical carbon, artificial graphite, and amorphous carbon.

From the viewpoint of capacity density, the negative electrode active material may be silicon (Si), tin (Sn), a silicon compound, or a tin compound. The silicon compound and the tin compound may each be an alloy or a solid solution.

An example of the silicon compound is SiOx (0.05<x<1.95). A compound (alloy or solid solution) obtained by partly substituting a silicon atom of SiOx with another element may also be used. The other element is at least one element selected from the group consisting of boron, magnesium, nickel, titanium, molybdenum, cobalt, calcium, chromium, copper, iron, manganese, niobium, tantalum, vanadium, tungsten, zinc, carbon, nitrogen, and tin.

Examples of the tin compound include Ni₂Sn₄, Mg₂Sn, SnO_(x) (0<x<2), SnO₂, and SnSiO₃. One tin compound selected from the foregoing may be used alone. Alternatively, two or more tin compounds selected from the foregoing may be used in combination.

The negative electrode active material may have any form. A negative electrode active material having a publicly known form (e.g., particulate form or fibrous form) can be used.

Lithium may be supplied (occluded) to the negative electrode active material layer 17 by any method. Specifically, the method is a method (a) in which lithium is deposited onto the negative electrode active material layer 17 by a gas phase method such as a vacuum deposition method or a method (b) in which a lithium metal foil and the negative electrode active material layer 17 are heated while being in contact with each other. By any of the methods, lithium can be diffused into the negative electrode active material layer 17 using heat. Alternatively, lithium may be electrochemically occluded into the negative electrode active material layer 17. Specifically, a battery is assembled using a negative electrode 22 not containing lithium and a lithium metal foil (positive electrode). Then, the battery is charged so that lithium is occluded into the negative electrode 22.

Examples of the binding agent for the positive electrode 21 and the negative electrode 22 include polyvinylidene fluoride, polytetrafluoroethylene, polyethylene, polypropylene, aramid resin, polyamide, polyimide, polyamide-imide, polyacrylonitrile, polyacrylic acid, poly(methyl acrylate), poly(ethyl acrylate), poly(hexyl acrylate), polymethacrylic acid, poly(methyl methacrylate), poly(ethyl methacrylate), poly(hexyl methacrylate), polyvinyl acetate, polyvinylpyrrolidone, polyether, polyether sulfone, hexafluoropolypropylene, styrene-butadiene rubber, and carboxymethyl cellulose. Alternatively, the binding agent may be a copolymer of two or more materials selected from the group consisting of tetrafluoroethylene, hexafluoroethane, hexafluoropropylene, perfluoroalkyl vinyl ethers, vinylidene fluoride, chlorotrifluoroethylene, ethylene, propylene, pentafluoropropylene, fluoromethyl vinyl ether, acrylic acid, and hexadiene. The binding agent may be a mixture of two or more materials selected from the foregoing materials.

Examples of the conductive agent for the positive electrode 21 and the negative electrode 22 include graphite, carbon black, conductive fibers, graphite fluoride, metal powder, conductive whiskers, conductive metal oxides, and organic conductive materials. Examples of the graphite include natural graphite and artificial graphite. Examples of the carbon black include acetylene black, Ketjenblack (registered trademark), channel black, furnace black, lamp black, and thermal black. An example of the metal powder is an aluminum powder. Examples of the conductive whiskers include zinc oxide whiskers and potassium titanate whiskers. An example of the conductive metal oxides is titanium oxide. An example of the organic conductive materials is a phenylene derivative.

The surface of the binding agent may be covered with a material used as the conductive agent. For example, the surface of the binding agent may be covered with carbon black. This increases the capacity of the battery.

The separator 14 may be made of a material having high ion permeability and sufficient mechanical strength. Examples of the material include microporous thin films, woven fabric, and nonwoven fabric. Specifically, the separator 14 is desirably made of a polyolefin such as polypropylene or polyethylene. The separator 14 made of a polyolefin has not only high durability, but also a shutdown function exhibited when excessively heated. The separator 14 has a thickness of, for example, 10 to 300 μm (or 10 to 40 μm). The separator 14 may be a single-layer film made of one material. Alternatively, the separator 14 may be a composite film (multilayer film) made of two or more materials. The separator 14 has a porosity of, for example, 30% to 70% (or 35% to 60%). The term “porosity” refers to a proportion of the volume of pores to the total volume of the separator 14. The “porosity” is measured by, for example, mercury intrusion.

The nonaqueous electrolytic solution contains a nonaqueous solvent and a lithium salt dissolved in the nonaqueous solvent.

Examples of the nonaqueous solvent include cyclic carbonate solvents, linear carbonate solvents, cyclic ether solvents, linear ether solvents, cyclic ester solvents, linear ester solvents, and fluorinated solvents.

Examples of the cyclic carbonate solvents include ethylene carbonate, propylene carbonate, and butylene carbonate.

Examples of the linear carbonate solvents include dimethyl carbonate, ethyl methyl carbonate, and diethyl carbonate.

Examples of the cyclic ether solvents include tetrahydrofuran, 1,4-dioxane, and 1,3-dioxolane.

Examples of the linear ether solvents include 1,2-dimethoxyethane and 1,2-diethoxyethane.

An example of the cyclic ester solvents is γ-butyrolactone.

An example of the linear ester solvents is methyl acetate.

Examples of the fluorinated solvents include fluoroethylene carbonate, methyl fluoropropionate, fluorobenzene, fluoroethyl methyl carbonate, and fluorodimethylene carbonate.

One nonaqueous solvent selected from the foregoing solvents may be used alone. Alternatively, two or more nonaqueous solvents selected from the foregoing solvents may be used in combination.

The nonaqueous electrolytic solution may contain at least one fluorinated solvent selected from the group consisting of fluoroethylene carbonate, methyl fluoropropionate, fluorobenzene, fluoroethyl methyl carbonate, and fluorodimethylene carbonate.

When these fluorinated solvents are contained in the nonaqueous electrolytic solution, the oxidation resistance of the nonaqueous electrolytic solution is improved.

As a result, even when the battery 10 is charged at high voltage, the battery 10 can be stably operated.

In the battery according to the second embodiment, the electrolyte may be a solid electrolyte.

Examples of the solid electrolyte include organic polymer solid electrolytes, oxide solid electrolytes, and sulfide solid electrolytes.

An example of the organic polymer solid electrolytes is a compound of a polymer compound and a lithium salt.

The polymer compound may have an ethylene oxide structure. In this case, a large amount of the lithium salt can be contained, which further improves the ionic conductivity.

Examples of the oxide solid electrolytes include NASICON solid electrolytes such as LiTi₂(PO₄)₃ and its element-substituted derivatives; (LaLi)TiO₃-based perovskite solid electrolytes; LISICON solid electrolytes such as Li₁₄ZnGe₄O₁₆, Li₄SiO₄, LiGeO₄, and their element-substituted derivatives; garnet solid electrolytes such as Li₇La₃Zr₂O₁₂ and its element-substituted derivatives; Li₃N and its H-substituted derivatives; and Li₃PO₄ and its N-substituted derivatives.

Examples of the sulfide solid electrolytes include Li₂S—P₂S₅, Li₂S—SiS₂, Li₂S—B₂S₃, Li₂S—GeS₂, Li_(3.25)Ge_(0.25)P_(0.75)S₄, and Li₁₀GeP₂S₁₂. Furthermore, LiX (X: F, Cl, Br, or I), Mo_(y), Li_(x)MO_(y) (M: P, Si, Ge, B, Al, Ga, or In) (x and y: natural number), or the like may be added to the foregoing sulfide solid electrolytes.

In particular, among these solid electrolytes, sulfide solid electrolytes have excellent formability and high ionic conductivity. Therefore, the use of the sulfide solid electrolytes can provide batteries having a higher energy density.

Among the sulfide solid electrolytes, Li₂S—P₂S₅ has high electrochemical stability and higher ionic conductivity. Therefore, the use of Li₂S—P₂S₅ as a solid electrolyte can provide batteries having a higher energy density.

A solid electrolyte layer may contain the above-described nonaqueous electrolytic solution.

When a solid electrolyte layer contains the nonaqueous electrolytic solution, lithium ion transfer is facilitated between the active material and the solid electrolyte. This can provide batteries having a higher energy density.

The solid electrolyte layer may contain, for example, a gel electrolyte and an ionic liquid in addition to the solid electrolyte.

The gel electrolyte may be a polymer material containing a nonaqueous electrolytic solution. Examples of the polymer material include polyethylene oxide, polyacrylonitrile, polyvinylidene fluoride, poly(methyl methacrylate), and polymers having an ethylene oxide bond.

Examples of cations for the ionic liquid include chain aliphatic quaternary salts such as tetraalkylammoniums and tetraalkylphosphoniums; alicyclic ammoniums such as pyrrolidiniums, morpholiniums, imidazoliniums, tetrahydropyrimidiniums, piperaziniums, and piperidiniums; and nitrogen-containing heterocyclic aromatic cations such as pyridiniums and imidazoliums. Examples of anions for the ionic liquid include PF₆ ⁻, BF₄ ⁻, SbF₆ ⁻, AsF₆ ⁻, SO₃CF₃ ⁻, N(SO₂CF₃)₂ ⁻, N(SO₂C₂F₅)₂ ⁻, N(SO₂CF₃)(SO₂C₄F₉)⁻, and C(SO₂CF₃)₃ ⁻. The ionic liquid may contain a lithium salt.

Examples of the lithium salt include LiPF₆, LiBF₄, LiSbF₆, LiAsF₆, LiSO₃CF₃, LiN(SO₂CF₃)₂, LiN(SO₂C₂F₅)₂, LiN(SO₂CF₃)(SO₂C₄F₉), and LiC(SO₂CF₃)₃. One lithium salt selected from the foregoing lithium salts may be used alone. Alternatively, two or more lithium salts selected from the foregoing lithium salts may be used as a mixture. The concentration of the lithium salt is, for example, in the range of 0.5 to 2 mol/L.

The battery according to the second embodiment may have various forms. For example, coin batteries, cylinder batteries, prismatic batteries, sheet batteries, button batteries, flat batteries, and stack batteries may be employed.

EXAMPLES Example 1 Production of Positive Electrode Active Material

LiF, Li₂MnO₃, and LiMnO₂ were weighed so as to have a molar ratio of Li/Mn/O/F=1.2/0.8/1.67/0.33.

The weighed raw materials were inserted into a 45 cc zirconia container together with an appropriate amount of ϕ3 mm zirconia balls, and the container was hermetically sealed in an argon glove box.

The raw materials were then taken out of the argon glove box and treated with a planetary ball mill at 600 rpm for 30 hours to obtain a precursor.

The obtained precursor was subjected to powder X-ray diffractometry.

The space group of the obtained precursor was Fm-3m.

The obtained precursor was then heat-treated in an air atmosphere at 300° C. for 1 hour to obtain a lithium composite oxide.

The obtained lithium composite oxide was then surface-treated by an atomic layer deposition method. Specifically, trimethylaluminum and ozone are alternately stacked in a vacuum atmosphere at 150° C. to form an alumina (Al₂O₃) coating film on the surface of the lithium composite oxide.

The produced positive electrode active material was subjected to powder X-ray diffractometry.

FIG. 2 illustrates the measurement results.

The space group of the obtained lithium composite oxide was found to be Fd-3m.

The integrated intensity ratio I₍₁₁₁₎/I₍₄₀₀₎ of a peak of a (111) plane to a peak of a (400) plane of the obtained lithium composite oxide was found to be 0.23.

As a result of the observation of the produced positive electrode active material through X-ray photoelectron spectroscopy, it was confirmed that Al₂O₃ was present on the surface of the lithium composite oxide represented by Li_(1.2)Mn_(0.8)O_(1.67)F_(0.33).

The Al₂O₃ coating film had a thickness of 0.5 nm.

Production of Battery

Subsequently, 70 parts by mass of the positive electrode active material, 20 parts by mass of a conductive agent, 10 parts by mass of polyvinylidene fluoride (PVDF), and an appropriate amount of 2-methylpyrrolidone (NMP) were mixed with each other. Thus, a positive electrode mixture slurry was prepared.

The positive electrode mixture slurry was applied onto one surface of a positive electrode current collector having a thickness of 20 μm and formed of an aluminum foil.

The positive electrode mixture slurry was dried and rolling was performed to obtain a positive electrode plate including a positive electrode active material layer and having a thickness of 60 μm.

The obtained positive electrode plate was stamped into a circular shape with a diameter of 12.5 mm to obtain a positive electrode.

Furthermore, a lithium metal foil having a thickness of 300 μm was stamped into a circular shape with a diameter of 14.0 mm to obtain a negative electrode.

Fluoroethylene carbonate (FEC), ethylene carbonate (EC), and ethyl methyl carbonate (EMC) were mixed at a volume ratio of 1:1:6 to obtain a nonaqueous solvent.

LiPF₆ was dissolved in the nonaqueous solvent in a concentration of 1.0 mol/L to obtain a nonaqueous electrolytic solution.

A separator (manufactured by Celgard, LLC., product number: 2320, thickness: 25 μm) was impregnated with the obtained nonaqueous electrolytic solution. The separator is a three-layer separator including a polypropylene layer, a polyethylene layer, and a polypropylene layer.

The positive electrode, the negative electrode, and the separator were assembled in a dry box in which the dew point was controlled to −50° C., and thus a CR2032 coin battery was produced.

Examples 2 to 4

The thickness of the Al₂O₃ coating film was changed from that in Example 1

Table 1 shows the thickness of each of Al₂O₃ coating films in Examples 2 to 4. The thickness of the Al₂O₃ coating film was 0.2 nm in Example 2, 1.0 nm in Example 3, and 2.0 nm in Example 4.

Except for this, positive electrode active materials in Examples 2 to 4 were synthesized in the same manner as in Example 1.

Furthermore, coin batteries in Examples 2 to 4 were produced in the same manner as in Example 1 using the positive electrode active materials in Examples 2 to 4.

Examples 5 to 7

The covering material added was changed from that in Example 1.

Table 1 shows the composition of each of covering materials used in Examples 5 to 7. ZrO₂ was used in Example 5, ZnO was used in Example 6, and TiO₂ was used in Example 7.

Except for this, positive electrode active materials in Examples 5 to 7 were synthesized in the same manner as in Example 1.

Furthermore, coin batteries in Examples 5 to 7 were produced in the same manner as in Example 1 using the positive electrode active materials in Examples 5 to 7.

Example 8

The composition of the lithium composite oxide was changed from that in Example 1.

Table 1 shows the composition of a lithium composite oxide used in Example 8. In Example 8, Li_(1.2)Mn_(0.6)Co_(0.1)Ni_(0.1)O_(1.67)F_(0.33) was used.

Except for this, a positive electrode active material in Example 8 was synthesized in the same manner as in Example 1.

A coin battery in Example 8 was produced in the same manner as in Example 1 using the positive electrode active material in Example 8.

Comparative Examples 1 and 2

Lithium composite oxides in Comparative Examples 1 and 2 were obtained in the same manner as in Examples 1 to 8.

Table 1 shows the composition of each of the lithium composite oxides in Comparative Examples 1 and 2.

Herein, the covering material was not added.

Except for this, positive electrode active materials in Comparative Examples 1 and 2 were synthesized in the same manner as in Example 1.

Coin batteries in Comparative Examples 1 and 2 were produced in the same manner as in Example 1 using the obtained positive electrode active materials.

Evaluation of Battery

The current density at the positive electrode was set to 0.5 mA/cm² and the batteries in Examples 1 to 8 and Comparative Examples 1 and 2 were charged until the voltage reached 4.9 V.

Subsequently, the end-of-discharge voltage was set to 2.5 V and the batteries in Examples 1 to 8 and Comparative Examples 1 and 2 were discharged at a current density of 0.5 mA/cm².

Furthermore, 20 cycles of the charge-discharge test were repeatedly performed to measure the capacity retention of each of the batteries in Examples 1 to 8 and Comparative Examples 1 and 2.

FIG. 3 illustrates the results of the initial charge-discharge test of the batteries in Example 1, Example 3, and Comparative Example 1.

FIG. 4 illustrates the results of the charge-discharge cycle test of the batteries in Example 1, Example 3, and Comparative Example 1.

The initial discharge capacity of the battery in Example 1 was 309 mAh/g. The initial discharge capacity of the battery in Example 3 was 294 mAh/g. On the other hand, the initial discharge capacity of the battery in Comparative Example 1 was 283 mAh/g.

The capacity retention after 20 cycles of the battery in Example 1 was 86%. The capacity retention after 20 cycles of the battery in Example 3 was 85%. On the other hand, the capacity retention after 20 cycles of the battery in Comparative Example 1 was 82%.

Table 1 show the results.

TABLE 1 Initial Lithium composite oxide Covering material discharge Capacity Space Com Thickness capacity retention Composition group I₍₁₁₁₎/I₍₄₀₀₎ position (nm) (mAh/g) (%) Example 1 Li_(1.2)Mn_(0.8)O_(1.67)F_(0.33) Fd-3m 0.23 Al₂O₃ 0.5 309 86 Example 2 Li_(1.2)Mn_(0.8)O_(1.67)F_(0.33) Fd-3m 0.24 Al₂O₃ 0.2 287 84 Example 3 Li_(1.2)Mn_(0.8)O_(1.67)F_(0.33) Fd-3m 0.24 Al₂O₃ 1.0 294 85 Example 4 Li_(1.2)Mn_(0.8)O_(1.67)F_(0.33) Fd-3m 0.25 Al₂O₃ 2.0 254 94 Example 5 Li_(1.2)Mn_(0.8)O_(1.67)F_(0.33) Fd-3m 0.27 ZrO₂ 0.5 303 83 Example 6 Li_(1.2)Mn_(0.8)O_(1.67)F_(0.33) Fd-3m 0.23 ZnO 0.5 301 83 Example 7 Li_(1.2)Mn_(0.8)O_(1.67)F_(0.33) Fd-3m 0.24 TiO₂ 0.5 300 84 Example 8 Li_(1.2)Mn_(0.6)Co_(0.1)Ni_(0.1)O_(1.67)F_(0.33) Fd-3m 0.28 A₂O₃ 0.5 298 85 Comparative Li_(1.2)Mn_(0.8)O_(1.67)F_(0.33) Fd-3m 0.23 — — 283 82 Example 1 Comparative Li_(1.2)Mn_(0.6)Co_(0.1)Ni_(0.1)O_(1.67)F_(0.33) Fd-3m 0.30 — — 280 81 Example 2

As shown in Table 1, the capacity retention after 20 cycles of the batteries in Examples 1 to 7 is higher than the capacity retention after 20 cycles of the battery in Comparative Example 1.

The reason for this is believed to be as follows. The batteries in Examples 1 to 7 contain a covering material. That is, the direct contact between the surface of the lithium composite oxide and the electrolytic solution is suppressed by the covering material. This suppresses, for example, the generation of gas due to a side reaction of the lithium composite oxide and the electrolytic solution, the elimination of oxygen, and the generation of a side reaction product onto the surface of the positive electrode active material. Thus, the capacity retention after 20 cycles is improved.

As shown in Table 1, the initial discharge capacity of the batteries in Examples 1 to 3 and Examples 5 to 7 is higher than the initial discharge capacity of the battery in Comparative Example 1.

The reason for this is believed to be as follows. The batteries in Examples 1 to 3 and Examples 5 to 7 contain a covering material. That is, the direct contact between the surface of the lithium composite oxide and the electrolytic solution is suppressed by the covering material. This suppresses the elimination of oxygen that leads to a reduction in the capacity of the positive electrode active material, which ameliorates the irreversibility of the charge-discharge reaction. Thus, the initial discharge capacity is improved.

As shown in Table 1, the initial discharge capacity of the battery in Example 4 is lower than that of the battery in Example 1.

As shown in Table 1, the capacity retention after 20 cycles of the battery in Example 4 is higher than that of the battery in Example 1.

The reason for this is believed to be as follows. The coating film of the covering material of the battery is thicker in Example 4 than in Example 1. This electrochemically deactivates the surface of the lithium composite oxide and increases the resistance, which decreases the Li conductivity. Thus, the initial discharge capacity is decreased. On the other hand, for example, the generation of gas due to a side reaction of the lithium composite oxide and the electrolytic solution, the elimination of oxygen, and the generation of a side reaction product onto the surface of the positive electrode active material are further suppressed. Thus, the capacity retention after 20 cycles is improved.

As shown in Table 1, the initial discharge capacity and capacity retention after 20 cycles of the battery in Example 1 are higher than those of the batteries in Examples 5 to 7.

The reason for this is believed to be as follows. In the battery in Example 1, Al₂O₃ is used as a covering material. As a result, Al₂O₃ reacts with Li on the surface of the lithium composite oxide, thereby forming LiAlO₂. This improves the Li conductivity. Thus, the initial discharge capacity and the capacity retention after 20 cycles are improved.

As shown in Table 1, the initial discharge capacity and capacity retention after 20 cycles of the battery in Example 8 are higher than those of the battery in Comparative Example 2.

The reason for this is believed to be as follows. The battery in Example 8 contains a covering material. That is, even when any lithium composite oxide according to the present disclosure other than the lithium composite oxides having the chemical compositions in Examples 1 to 8 is used, the cycle characteristics are improved by adding a covering material.

Hereafter, Reference Examples will be described. The positive electrode active materials in Reference Examples below contain a lithium composite oxide but not the covering material according to the present disclosure.

Reference Example 1

LiF, Li₂MnO_(3,) and LiMnO₂ were weighed so as to have a molar ratio of Li/Mn/O/F=1.2/0.8/1.67/0.33.

The weighed raw materials were inserted into a 45 cc zirconia container together with an appropriate amount of ϕ3 mm zirconia balls, and the container was hermetically sealed in an argon glove box.

The raw materials were then taken out of the argon glove box and treated with a planetary ball mill at 600 rpm for 30 hours to obtain a precursor.

The obtained precursor was subjected to powder X-ray diffractometry.

The space group of the obtained precursor was Fm-3m.

The obtained precursor was then heat-treated in an air atmosphere at 300° C. for 1 hour.

The produced positive electrode active material was subjected to powder X-ray diffractometry.

The space group of the produced positive electrode active material was Fd-3m.

The integrated intensity ratio I₍₁₁₁₎/I₍₄₀₀₎ of a peak of a (111) plane to a peak of a (400) plane of the produced positive electrode active material was 0.23.

A coin battery in Reference Example 1 was produced in the same manner as in Example 1 using the produced positive electrode active material.

Reference Examples 2 to 19

The raw materials and the mixing ratio of Li/Me/O/F were changed from those in Reference Example 1.

Furthermore, the heat treatment conditions were changed within 200° C. to 600° C. and 5 minutes to 20 hours from those in Reference Example 1.

Except for this, positive electrode active materials in Reference Examples 2 to 19 were synthesized in the same manner as in Reference Example 1.

Table 2 shows the composition of each of the positive electrode active materials in Reference Examples 2 to 19.

The space group of each of the positive electrode active materials in Reference Examples 2 to 19 was Fd-3m.

The raw materials in Reference Examples 2 to 19 were weighed on a stoichiometric basis and mixed with each other in the same manner as in Reference Example 1.

For example, in Reference Example 4, LiF, Li₂MnO₃, LiMnO₂, LiCoO₂, and LiNiO2 were weighed so as to have a molar ratio of Li/Mn/Co/Ni/O/F=1.2/0.6/0.1/0.1/1.67/0.33 and mixed with each other.

Furthermore, coin batteries in Reference Examples 2 to 19 were produced in the same manner as in Reference Example 1 using the positive electrode active materials in Reference Examples 2 to 19.

Reference Example 20

A positive electrode active material having a composition represented by Li_(1.2)Mn_(0.8)O₂ was produced in the same manner as in Reference Example 1.

Herein, LiF was not used as a raw material.

The space group of the produced positive electrode active material was Fd-3m.

The integrated intensity ratio I₍₁₁₁₎/I₍₄₀₀₎ of a peak of a (111) plane to a peak of a (400) plane of the produced positive electrode active material was 0.15.

A coin battery in Reference Example 20 was produced in the same manner as in Reference Example 1 using the produced positive electrode active material.

Reference Example 21

A positive electrode active material having a composition represented by LiMn₂O₄ (lithium manganese oxide) was produced by a publicly known method.

The produced positive electrode active material was subjected to powder X-ray diffractometry.

The space group of the produced positive electrode active material was Fd-3m.

The integrated intensity ratio I₍₁₁₁₎/I₍₄₀₀₎ of a peak of a (111) plane to a peak of a (400) plane of the produced positive electrode active material was 1.30.

A coin battery in Reference Example 21 was produced in the same manner as in Reference Example 1 using the produced positive electrode active material.

Reference Example 22

A positive electrode active material having a composition represented by Li1.2Mn_(0.8)O_(1.67)F_(0.33) was produced in the same manner as in Reference Example 1.

Herein, the heat treatment conditions were changed to 500° C. and 1 hour.

The space group of the positive electrode active material was Fd-3m.

The integrated intensity ratio I₍₁₁₁₎/I₍₄₀₀₎ of a peak of a (111) plane to a peak of a (400) plane of the produced positive electrode active material was 1.04.

A coin battery in Reference Example 22 was produced in the same manner as in Reference Example 1 using the produced positive electrode active material.

Reference Example 23

A positive electrode active material having a composition represented by Li1.2Mn_(0.8)O_(1.67)F0.33 was produced in the same manner as in Reference Example 1.

Herein, the heat treatment conditions were changed to 300° C. and 5 minutes.

The space group of the produced positive electrode active material was Fd-3m.

The integrated intensity ratio I₍₁₁₁₎/I₍₄₀₀₎ of a peak of a (111) plane to a peak of a (400) plane of the produced positive electrode active material was 0.02.

A coin battery in Reference Example 23 was produced in the same manner as in Reference Example 1 using the produced positive electrode active material.

Evaluation of Battery

The current density at the positive electrode was set to 0.5 mA/cm² and the battery in Reference Example 1 was charged until the voltage reached 4.9 V.

Subsequently, the end-of-discharge voltage was set to 2.5 V and the battery in Reference Example 1 was discharged at a current density of 0.5 mA/cm².

The initial discharge capacity of the battery in Reference Example 1 was 300 mAh/g.

Furthermore, the current density at the positive electrode was set to 0.5 mA/cm² and the battery in Reference Example 21 was charged until the voltage reached 4.3 V.

Subsequently, the end-of-discharge voltage was set to 2.5 V and the battery in Reference Example 21 was discharged at a current density of 0.5 mA/cm².

The initial discharge capacity of the battery in Reference Example 21 was 140 mAh/g.

Furthermore, the initial discharge capacity of each of the coin batteries in Reference Examples 2 to 20 and Reference Examples 22 and 23 was measured.

Table 2 shows the results.

TABLE 2 Initial discharge (x + y)/ Space capacity Composition x/y α/β (α/β) group I₍₁₁₁₎/I₍₄₀₀₎ (mAh/g) Reference Li_(1.2)Mn_(0.8)O_(1.67)F_(0.33) 1.5 5 1.0 Fd-3m 0.23 300 Example 1 Reference Li_(1.2)Mn_(0.8)O_(1.67)F_(0.33) 1.5 5 1.0 Fd-3m 0.05 284 Example 2 Reference Li_(1.2)Mn_(0.8)O_(1.67)F_(0.33) 1.5 5 1.0 Fd-3m 0.70 272 Example 3 Reference Li_(1.2)Mn_(0.6)Co_(0.1)Ni_(0.1)O_(1.67)F_(0.33) 1.5 5 1.0 Fd-3m 0.30 287 Example 4 Reference Li_(1.2)Mn_(0.4)Co_(0.2)Ni_(0.2)O_(1.67)F_(0.33) 1.5 5 1.0 Fd-3m 0.25 275 Example 5 Reference Li_(1.2)Mn_(0.8)O_(1.9)F_(0.1) 1.5 19 1.0 Fd-3m 0.25 292 Example 6 Reference Li_(1.2)Mn_(0.8)O_(1.67)F_(0.33) 1.38 5 0.95 Fd-3m 0.29 297 Example 7 Reference Li_(1.2)Mn_(0.75)B_(0.05)O_(1.67)F_(0.33) 1.5 5 1.0 Fd-3m 0.28 285 Example 8 Reference Li_(1.2)Mn_(0.75)P_(0.05)O_(1.67)F_(0.33) 1.5 5 1.0 Fd-3m 0.24 289 Example 9 Reference Li_(1.2)Mn_(0.75)Al_(0.05)O_(1.67)F_(0.33) 1.5 5 1.0 Fd-3m 0.23 282 Example 10 Reference Li_(1.2)Mn_(0.75)Ti_(0.05)O_(1.67)F_(0.33) 1.5 5 1.0 Fd-3m 0.23 280 Example 11 Reference Li_(1.2)Mn_(0.75)Nb_(0.05)O_(1.67)F_(0.33) 1.5 5 1.0 Fd-3m 0.19 277 Example 12 Reference Li_(1.2)Mn_(0.75)W_(0.05)O_(1.67)F_(0.33) 1.5 5 1.0 Fd-3m 0.20 276 Example 13 Reference Li_(1.2)Mn_(0.75)V_(0.05)O_(1.67)F_(0.33) 1.5 5 1.0 Fd-3m 0.22 281 Example 14 Reference Li_(1.2)Mn_(0.75)Cr_(0.05)O_(1.67)F_(0.33) 1.5 5 1.0 Fd-3m 0.22 283 Example 15 Reference Li_(1.2)Mn_(0.75)Si_(0.05)O_(1.67)F_(0.33) 1.5 5 1.0 Fd-3m 0.24 275 Example 16 Reference Li_(1.2)Mn_(0.75)Fe_(0.05)O_(1.67)F_(0.33) 1.5 5 1.0 Fd-3m 0.19 276 Example 17 Reference Li_(1.2)Mn_(0.75)Cu_(0.05)O_(1.67)F_(0.33) 1.5 5 1.0 Fd-3m 0.22 274 Example 18 Reference Li_(1.2)Mn_(0.75)Ru_(0.05)O_(1.67)F_(0.33) 1.5 5 1.0 Fd-3m 0.21 278 Example 19 Reference Li_(1.2)Mn_(0.8)O₂ 1.5 — 1.0 Fd-3m 0.15 267 Example 20 Reference LiMn₂O₄ 0.5 — 0.75 Fd-3m 1.30 140 Example 21 Reference Li_(1.2)Mn_(0.8)O_(1.67)F_(0.33) 1.5 5 1.0 Fd-3m 1.04 238 Example 22 Reference Li_(1.2)Mn_(0.8)O_(1.67)F_(0.33) 1.5 5 1.0 Fd-3m 0.02 251 Example 23

As shown in Table 2, the batteries in Reference Examples 1 to 20 have an initial discharge capacity of 267 to 300 mAh/g.

That is, the initial discharge capacity of the batteries in Reference Examples 1 to 20 is higher than the initial discharge capacity of the batteries in Reference Examples 21 to 23.

The reason for this is believed to be as follows. In the batteries in Reference Examples 1 to 20, the lithium composite oxide of the positive electrode active material has a crystal structure that belongs to space group Fd-3m and the integrated intensity ratio I₍₁₁₁₎/I₍₄₀₀₎ of a peak of a (111) plane to a peak of a (400) plane in an XRD pattern satisfies 0.05≤I₍₁₁₁₎/I₍₄₀₀₎≤0.90. That is, when Li and Me are sufficiently subjected to cation mixing, the number of adjacent Li atoms increases, which improves the diffusibility of Li. This increases the number of three-dimensional diffusion paths of lithium, and a larger amount of Li can be intercalated and deintercalated. Thus, the initial discharge capacity is considerably improved.

In Reference Example 21, the ratio I₍₁₁₁₎/I₍₄₀₀₎ is more than 0.90 (I₍₁₁₁₎/I₍₄₀₀₎=1.30). Therefore, it is believed that the cation mixing is suppressed, which reduces the number of three-dimensional diffusion paths of lithium. Furthermore, the ratio x/y is small (x/y=0.5). Therefore, it is believed that the amount of Li that contributes to the reaction is small, which degrades the diffusibility of Li ions. Thus, the initial discharge capacity is believed to be considerably decreased.

In Reference Example 22, the ratio I₍₁₁₁₎/I₍₄₀₀₎ is more than 0.90 (I₍₁₁₁₎/I₍₄₀₀₎=1.04). Therefore, it is believed that the cation mixing is suppressed, which reduces the number of three-dimensional diffusion paths of lithium. Thus, the initial discharge capacity is believed to be decreased.

In Reference Example 23, the ratio I₍₁₁₁₎/I₍₄₀₀₎ is less than 0.05 (I₍₁₁₁₎/I₍₄₀₀₎=0.02). Therefore, it is believed that the crystal structure becomes thermodynamically unstable and is broken when the deintercalation of Li during charge proceeds. Thus, the initial discharge capacity is believed to be decreased.

As shown in Table 2, the initial discharge capacity of the battery in Reference Example 2 is lower than the initial discharge capacity of the battery in Reference Example 1.

The reason for this is believed to be as follows. The ratio I₍₁₁₁₎/I₍₄₀₀₎ in Reference Example 2 (I₍₁₁₁₎/l(400)=0.05) is smaller than that in Reference Example 1. Therefore, the crystal structure becomes unstable, which decreases the initial discharge capacity.

As shown in Table 2, the initial discharge capacity of the battery in Reference Example 3 is lower than the initial discharge capacity of the battery in Reference Example 1.

The reason for this is believed to be as follows. The ratio I₍₁₁₁₎/I₍₄₀₀₎ in Reference Example 3 (I₍₁₁₁₎/I₍₄₀₀₎=0.70) is larger than that in Reference Example 1. Therefore, the cation mixing is suppressed, which slightly reduces the number of three-dimensional diffusion paths of lithium. Thus, the initial discharge capacity is decreased.

As shown in Table 2, the initial discharge capacity of the battery in Reference Example 4 is lower than the initial discharge capacity in Reference Example 1.

The reason for this is believed to be as follows. In Reference Example 4, the amount of Mn having orbitals easily hybridized with those of oxygen is decreased compared with in Reference Example 1. That is, Co and Ni are less easily subjected to orbital hybridization with oxygen than Mn. Therefore, oxygen is eliminated during charge and the crystal structure becomes unstable. Furthermore, the contribution of oxygen to the oxidation-reduction reaction decreases. Thus, the initial discharge capacity is decreased.

As shown in Table 2, the initial discharge capacity of the battery in Reference Example 5 is lower than the initial discharge capacity of the battery in Reference Example 4.

The reason for this is believed to be as follows. In Reference Example 5, the amount of Mn having orbitals easily hybridized with those of oxygen is decreased compared with in Reference Example 4. Therefore, oxygen is eliminated during charge and the crystal structure becomes unstable. Furthermore, the contribution of oxygen to the oxidation-reduction reaction decreases. Thus, the initial discharge capacity is decreased.

As shown in Table 2, the initial discharge capacity of the battery in Reference Example 6 is lower than the initial discharge capacity of the battery in Reference Example 1.

The reason for this is believed to be as follows. The ratio α/β in Reference Example 6 (α/β=19) is larger than that in Reference Example 1. That is, the capacity due to the oxidation-reduction reaction of oxygen excessively increases. Furthermore, the influence of F, which has a high electronegativity, is reduced and the structure becomes unstable when the deintercalation of Li proceeds. Thus, the initial discharge capacity is decreased.

As shown in Table 2, the initial discharge capacity of the battery in Reference Example 7 is lower than the initial discharge capacity of the battery in Reference Example 1.

The reason for this is believed to be as follows. The ratio x/y in Reference Example 7 (x/y=1.38) is smaller than that in Reference Example 1. Therefore, the number of isolated lithium atoms increases in the crystal structure, which decreases the amount of Li that contributes to the reaction. This degrades the diffusibility of Li ions and decreases the initial discharge capacity. On the other hand, the isolated lithium atoms function as supporting pillars and thus the cycle characteristics are improved.

As shown in Table 2, the initial discharge capacity of the batteries in Reference Examples 8 to 19 is lower than the initial discharge capacity of the battery in Reference Example 1.

The reason for this is believed to be as follows. In Reference Examples 8 to 19, the amount of Mn having orbitals easily hybridized with those of oxygen is decreased compared with in Reference Example 1. This decreases the contribution of oxygen to the oxidation-reduction reaction, and thus the initial discharge capacity is decreased.

As shown in Table 2, the initial discharge capacity of the battery in Reference Example 20 is lower than the initial discharge capacity of the battery in Reference Example 1.

The reason for this is believed to be as follows. In Reference Example 20, the lithium composite oxide does not contain F. Therefore, oxygen is not partly replaced with F, which has a high electronegativity, and thus the cation-anion interaction is decreased. This destabilizes the crystal structure through the elimination of oxygen during charge. Thus, the initial discharge capacity is decreased. 

What is claimed is:
 1. A positive electrode active material comprising: a lithium composite oxide; and a covering material that covers a surface of the lithium composite oxide, wherein the covering material has an electron conductivity of 10⁶ S/m or less, the lithium composite oxide has a crystal structure that belongs to space group Fd-3m, and a ratio I₍₁₁₁₎/I₍₄₀₀₎ of a first integrated intensity I₍₁₁₁₎ of a first peak corresponding to a (111) plane to a second integrated intensity I₍₄₀₀₎ of a second peak corresponding to a (400) plane in an XRD pattern of the lithium composite oxide satisfies 0.05≤I₍₁₁₁₎/I₍₄₀₀₎≤0.90.
 2. The positive electrode active material according to claim 1, wherein the covering material is an inorganic compound.
 3. The positive electrode active material according to claim 1, wherein a mass ratio of the covering material to the lithium composite oxide is 0.2 or less.
 4. The positive electrode active material according to claim 3, wherein the mass ratio of the covering material to the lithium composite oxide is 0.01 or more and 0.1 or less.
 5. The positive electrode active material according to claim 1, wherein the covering material has a thickness of 0.1 nm or more and 2.0 nm or less.
 6. The positive electrode active material according to claim 1, wherein the covering material forms a solid solution with at least a part of the surface of the lithium composite oxide.
 7. The positive electrode active material according to claim 1, wherein the covering material is an oxide.
 8. The positive electrode active material according to claim 7, wherein the oxide is represented by a formula Li_(a)A_(b)O_(c), where A is at least one element selected from the group consisting of Mn, Co, Ni, Fe, Cu, Nb, Mo, Ti, Al, Cr, Zr, Zn, Na, K, Ca, Mg, Pt, Au, B, P, Eu, Sm, Ce, and H; and 0≤a≤3, 0.5≤b≤4, and 1≤c≤4.
 9. The positive electrode active material according to claim 8, wherein the oxide is at least one oxide selected from the group consisting of Al₂O₃, ZrO₂, ZnO, and TiO₂.
 10. The positive electrode active material according to claim 1, wherein 0.05≤I₍₁₁₁₎/I₍₄₀₀₎≤0.70.
 11. The positive electrode active material according to claim 10, wherein 0.05≤I₍₁₁₁₎/I₍₄₀₀₎≤0.30.
 12. The positive electrode active material according to claim 1, wherein the lithium composite oxide contains manganese.
 13. The positive electrode active material according to claim 1, wherein the lithium composite oxide contains fluorine.
 14. The positive electrode active material according to claim 1, wherein the lithium composite oxide is represented by a formula Li_(x)Me_(y)O_(α)Q_(β), where Me is at least one element selected from the group consisting of Mn, Co, Ni, Fe, Cu, V, Nb, Mo, Ti, Cr, Zr, Zn, Na, K, Ca, Mg, Pt, Au, Ag, Ru, W, B, Si, P, and Al; Q is at least one element selected from the group consisting of F, Cl, N, and S; and 1.05≤x≤1.4, 0.6≤y≤0.95, 1.33≤α≤2, and 0≤β≤0.67.
 15. The positive electrode active material according to claim 14, wherein 5≤α/β≤19.
 16. The positive electrode active material according to claim 1, comprising the lithium composite oxide as a main component.
 17. A battery comprising: a positive electrode containing the positive electrode active material according to claim 1; a negative electrode; and an electrolyte.
 18. The battery according to claim 17, wherein the negative electrode contains: a negative electrode active material that allows a lithium ion to be inserted therein and released therefrom; or a material that allows lithium metal to be dissolved and deposited thereon, and the electrolyte is a nonaqueous electrolytic solution.
 19. The battery according to claim 17, wherein the negative electrode contains: a negative electrode active material that allows a lithium ion to be inserted therein and released therefrom; or a material that allows lithium metal to be dissolved and deposited thereon, and the electrolyte is a solid electrolyte. 